Why would anyone install a central heating system without putting in
proper provision to drain it? It seems senseless to me. Are you 100%
sure that no other 'main' draincock exists outside?

It is quite common for the pipework below floor level not to have any
drainage ability. The same situation existings on my system and almost every
system I've worked on. The main difference between those cases is this is
that the pipework for me is below a suspended ground floor. It matters
little if you just cut the pipe and have the bottom dregs pour away, unlike
a flat with more accommodation below.

Christian.